Method and device for performing uplink transmission after receiving trigger frame in wireless lan system

ABSTRACT

One embodiment of the present invention relates to a method by which a station (STA) performs uplink transmission in a wireless communication system, the method for performing uplink transmission comprising the steps of: receiving a trigger frame from an AP; and performing uplink transmission as a response to the trigger frame, wherein, when a network allocation vector (NAV) of the STA receiving the trigger frame is idle and clear channel assessment (CCA) result is idle, the uplink transmission is performed.

TECHNICAL FIELD

Following description invention relates to a wireless communicationsystem, and more particularly, to a method of performing uplinktransmission after a trigger frame is received in a wireless LAN systemand an apparatus therefor.

BACKGROUND ART

With recent development of information communication technologies, avariety of wireless communication technologies have been developed.Among such technologies, WLAN allows wireless Internet access at home,in businesses, or in specific service providing areas using a mobileterminal, such as a personal digital assistant (PDA), a laptop computer,and a portable multimedia player (PMP), based on radio frequencytechnology.

In order to overcome limited communication speed, which has been pointedout as a weak point of WLAN, technical standards have recentlyintroduced a system capable of increasing the speed and reliability of anetwork while extending a coverage region of a wireless network. Forexample, IEEE 802.11n supports high throughput (HT) with a maximum dataprocessing speed of 540 Mbps. In addition, Multiple Input MultipleOutput (MIMO) technology, which employs multiple antennas for both atransmitter and a receiver in order to minimize transmission errors andoptimize data rate, has been introduced.

Machine-to-machine (M2M) communication technology has been discussed asa next generation communication technology. A technical standard tosupport M2M communication in the IEEE 802.11 WLAN system is also underdevelopment as IEEE 802.11ah. In M2M communication, a scenario in whicha small amount of data is occasionally communicated at a low speed in anenvironment including a large number of devices may be considered.

In a wireless LAN system, communication is performed in a medium sharedby all devices. If the number of devices increases in communication suchas M2M communication, it is necessary to more efficiently enhance achannel access mechanism to reduce unnecessary power consumption andinterference occurrence.

DISCLOSURE OF THE INVENTION Technical Task

A technical task of the present invention is to provide a method ofefficiently performing uplink transmission after a trigger frame isreceived.

Technical tasks obtainable from the present invention are non-limited bythe above-mentioned technical task. And, other unmentioned technicaltasks can be clearly understood from the following description by thosehaving ordinary skill in the technical field to which the presentinvention pertains.

Technical Solution

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, accordingto one embodiment, a method of performing uplink transmission, which isperformed by an STA (station) in a wireless communication system,includes the steps of receiving a trigger frame from an AP andperforming uplink transmission in response to the trigger frame. In thiscase, if an NAV (network allocation vector) of the STA, which havereceived the trigger frame, is idle and a CCA (clear channel assessment)result is idle, uplink transmission can be performed.

To further achieve these and other advantages and in accordance with thepurpose of the present invention, according to a different embodiment,an STA (station) performing uplink transmission in a wirelesscommunication system includes a transceiver and a processor, theprocessor configured to receive a trigger frame from an AP, theprocessor configured to perform uplink transmission in response to thetrigger frame. In this case, if an NAV (network allocation vector) ofthe STA, which have received the trigger frame, is idle and a CCA (clearchannel assessment) result is idle, uplink transmission can beperformed.

If the NAV of the STA is idle and one of a size of a resource allocatedby the trigger frame and a data size of the uplink transmission issmaller than a threshold value indicated by the trigger frame, the STAcan perform uplink transmission irrespective of the CCA result beforethe trigger frame is received.

The trigger frame may correspond to a frame for triggering uplinkmulti-user transmission.

If a NAV count of the STA corresponds to 0 or a non-bandwidth signalingTA of the trigger frame is identical to an address of a TXOP holderwhile the NAV count is not 0, the NAV is idle.

The threshold value indicated by the trigger frame may correspond to adecimal value *4 corresponding to bits transmitted via the trigger frameand a unit of the threshold value may correspond to Octet.

The threshold value indicated by the trigger frame may correspond to adecimal value corresponding to bits transmitted via the trigger frameand a unit of the threshold value may correspond to us.

The uplink transmission can be performed when SIFS (short inter-framespace) elapsed after the trigger frame is received.

The uplink transmission may transmit one selected from the groupconsisting of PS-Poll, Ack/Block Ack, Resource Request/Buffer Statusreport, CTS, and NDP frame.

Advantageous Effects

According to embodiments of the present invention, it is able to moreefficiently perform uplink transmission in consideration of such asituation as a blackout and transmission of a trigger frame.

Effects obtainable from the present invention are non-limited by theabove mentioned effect. And, other unmentioned effects can be clearlyunderstood from the following description by those having ordinary skillin the technical field to which the present invention pertains.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is a diagram for an example of a structure of IEEE 802.11 systemto which the present invention is applicable;

FIG. 2 is a diagram for a different example of a structure of IEEE802.11 system to which the present invention is applicable;

FIG. 3 is a diagram for a further different example of a structure ofIEEE 802.11 system to which the present invention is applicable;

FIG. 4 is a diagram for an example of a structure of a wireless LANsystem;

FIG. 5 is a diagram for explaining a link setup procedure in a wirelessLAN system;

FIG. 6 is a diagram for explaining a backoff procedure;

FIG. 7 is a diagram for explaining a hidden node and an exposed node;

FIG. 8 is a diagram for explaining RTS and CTS;

FIG. 9 is a diagram for explaining a power management operation;

FIGS. 10 to 12 are diagrams for explaining an operation of an STA, whichhas received TIM, in detail;

FIG. 13 is a diagram for an example of an MAC frame format of IEEE802.11 system to which the present invention is applicable;

FIG. 14 is a diagram for an example of an HT format of an HT controlfield in an MAC frame;

FIG. 15 is a diagram for an example of a VHT format of an HT controlfield in an MAC frame;

FIG. 16 is a diagram for an example of a PPDU frame format of IEEE802.11n system;

FIGS. 17 and 18 are diagrams for an example of a VHT PPDU frame formatof IEEE 802.11ac system;

FIG. 19 is a diagram for an example of a PPDU format in 11ax;

FIG. 20 is a diagram for an example of 11ax MU PS poll;

FIG. 21 is a diagram for an example of a UL MU U-APSD operation;

FIG. 22 is a diagram for an example of transmitting a plurality oftrigger frames;

FIGS. 23 and 24 are diagrams for explaining embodiments of the presentinvention;

FIG. 25 is a block diagram for a configuration of a wireless deviceaccording to one embodiment of the present invention.

BEST MODE Mode for Invention

Hereinafter, exemplary embodiments of the present invention will bedescribed with reference to the accompanying drawings. The detaileddescription, which will be disclosed along with the accompanyingdrawings, is intended to describe exemplary embodiments of the presentinvention and is not intended to describe a unique embodiment throughwhich the present invention can be carried out. The following detaileddescription includes specific details in order to provide a thoroughunderstanding of the present invention. However, it will be apparent tothose skilled in the art that the present invention may be practicedwithout such specific details.

The embodiments of the present invention described hereinbelow arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions or features ofany one embodiment may be included in another embodiment and may bereplaced with corresponding constructions or features of anotherembodiment.

Specific terms used in the following description are provided to aid inunderstanding of the present invention. These specific terms may bereplaced with other terms within the scope and spirit of the presentinvention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention and theimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

The embodiments of the present invention can be supported by standarddocuments disclosed for at least one of wireless access systems such asthe institute of electrical and electronics engineers (IEEE) 802, 3rdgeneration partnership project (3GPP), 3GPP long term evolution (3GPPLTE), LTE-advanced (LTE-A), and 3GPP2 systems. For steps or parts ofwhich description is omitted to clarify the technical features of thepresent invention, reference may be made to these documents. Further,all terms as set forth herein can be explained by the standarddocuments.

The following technology can be used in various wireless access systemssuch as systems for code division multiple access (CDMA), frequencydivision multiple access (FDMA), time division multiple access (TDMA),orthogonal frequency division multiple access (OFDMA), single carrierfrequency division multiple access (SC-FDMA), etc. CDMA may beimplemented by radio technology such as universal terrestrial radioaccess (UTRA) or CDMA2000. TDMA may be implemented by radio technologysuch as global system for mobile communications (GSM)/general packetradio service (GPRS)/enhanced data rates for GSM evolution (EDGE). OFDMAmay be implemented by radio technology such as IEEE 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802.20, evolved-UTRA (E-UTRA), etc. For clarity,the present disclosure focuses on 3GPP LTE and LTE-A systems. However,the technical features of the present invention are not limited thereto.

Structure of WLAN System

FIG. 1 is a diagram showing an exemplary structure of an IEEE 802.11system to which the present invention is applicable.

The structure of the IEEE 802.11 system may include a plurality ofcomponents. A WLAN which supports transparent station (STA) mobility fora higher layer may be provided by mutual operations of the components. Abasic service set (BSS) may correspond to a basic building block in anIEEE 802.11 LAN. In FIG. 1, two BSSs (BSS1 and BSS2) are present and twoSTAs are included in each of the BSSs (i.e. STA1 and STA2 are includedin BSS1 and STA3 and STA4 are included in BSS2). An ellipse indicatingthe BSS in FIG. 1 may be understood as a coverage area in which STAsincluded in a corresponding BSS maintain communication. This area may bereferred to as a basic service area (BSA). If an STA moves out of theBSA, the STA cannot directly communicate with the other STAs in thecorresponding BSA.

In the IEEE 802.11 LAN, the most basic type of BSS is an independent BSS(IBSS). For example, the IBSS may have a minimum form consisting of onlytwo STAs. The BSS (BSS1 or BSS2) of FIG. 1, which is the simplest formand does not include other components except for the STAs, maycorrespond to a typical example of the IBSS. This configuration ispossible when STAs can directly communicate with each other. Such a typeof LAN may be configured as necessary instead of being prescheduled andis also called an ad-hoc network.

Memberships of an STA in the BSS may be dynamically changed when the STAbecomes an on or off state or the STA enters or leaves a region of theBSS. To become a member of the BSS, the STA may use a synchronizationprocess to join the BSS. To access all services of a BSS infrastructure,the STA should be associated with the BSS. Such association may bedynamically configured and may include use of a distributed systemservice (DSS).

FIG. 2 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In FIG. 2,components such as a distribution system (DS), a distribution systemmedium (DSM), and an access point (AP) are added to the structure ofFIG. 1.

A direct STA-to-STA distance in a LAN may be restricted by physical(PHY) performance. In some cases, such restriction of the distance maybe sufficient for communication. However, in other cases, communicationbetween STAs over a long distance may be necessary. The DS may beconfigured to support extended coverage.

The DS refers to a structure in which BSSs are connected to each other.Specifically, a BSS may be configured as a component of an extended formof a network consisting of a plurality of BSSs, instead of independentconfiguration as shown in FIG. 1.

The DS is a logical concept and may be specified by the characteristicof the DSM. In relation to this, a wireless medium (WM) and the DSM arelogically distinguished in IEEE 802.11. Respective logical media areused for different purposes and are used by different components. Indefinition of IEEE 802.11, such media are not restricted to the same ordifferent media. The flexibility of the IEEE 802.11 LAN architecture (DSarchitecture or other network architectures) can be explained in that aplurality of media is logically different. That is, the IEEE 802.11 LANarchitecture can be variously implemented and may be independentlyspecified by a physical characteristic of each implementation.

The DS may support mobile devices by providing seamless integration ofmultiple BSSs and providing logical services necessary for handling anaddress to a destination.

The AP refers to an entity that enables associated STAs to access the DSthrough a WM and that has STA functionality. Data can be moved betweenthe BSS and the DS through the AP. For example, STA2 and STA3 shown inFIG. 2 have STA functionality and provide a function of causingassociated STAs (STA1 and STA4) to access the DS. Moreover, since allAPs correspond basically to STAs, all APs are addressable entities. Anaddress used by an AP for communication on the WM need not necessarilybe identical to an address used by the AP for communication on the DSM.

Data transmitted from one of STAs associated with the AP to an STAaddress of the AP may be always received by an uncontrolled port and maybe processed by an IEEE 802.1X port access entity. If the controlledport is authenticated, transmission data (or frame) may be transmittedto the DS.

FIG. 3 is a diagram showing still another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In additionto the structure of FIG. 2, FIG. 3 conceptually shows an extendedservice set (ESS) for providing wide coverage.

A wireless network having arbitrary size and complexity may be comprisedof a DS and BSSs. In the IEEE 802.11 system, such a type of network isreferred to an ESS network. The ESS may correspond to a set of BSSsconnected to one DS. However, the ESS does not include the DS. The ESSnetwork is characterized in that the ESS network appears as an IBSSnetwork in a logical link control (LLC) layer. STAs included in the ESSmay communicate with each other and mobile STAs are movabletransparently in LLC from one BSS to another BSS (within the same ESS).

In IEEE 802.11, relative physical locations of the BSSs in FIG. 3 arenot assumed and the following forms are all possible. BSSs may partiallyoverlap and this form is generally used to provide continuous coverage.BSSs may not be physically connected and the logical distances betweenBSSs have no limit. BSSs may be located at the same physical positionand this form may be used to provide redundancy. One (or more than one)IBSS or ESS networks may be physically located in the same space as one(or more than one) ESS network. This may correspond to an ESS networkform in the case in which an ad-hoc network operates in a location inwhich an ESS network is present, the case in which IEEE 802.11 networksdifferent organizations physically overlap, or the case in which two ormore different access and security policies are necessary in the samelocation.

FIG. 4 is a diagram showing an exemplary structure of a WLAN system. InFIG. 4, an example of an infrastructure BSS including a DS is shown.

In the example of FIG. 4, BSS1 and BSS2 constitute an ESS. In the WLANsystem, an STA is a device operating according to MAC/PHY regulation ofIEEE 802.11. STAs include AP STAs and non-AP STAs. The non-AP STAscorrespond to devices, such as mobile phones, handled directly by users.In FIG. 4, STA1, STA3, and STA4 correspond to the non-AP STAs and STA2and STAS correspond to AP STAs.

In the following description, the non-AP STA may be referred to as aterminal, a wireless transmit/receive unit (WTRU), a user equipment(UE), a mobile station (MS), a mobile terminal, or a mobile subscriberstation (MSS). The AP is a concept corresponding to a base station (BS),a Node-B, an evolved Node-B (eNB), a base transceiver system (BTS), or afemto BS in other wireless communication fields.

Link Setup Process

FIG. 5 is a diagram for explaining a general link setup process.

In order to allow an STA to establish link setup on a network andtransmit/receive data over the network, the STA should perform processesof network discovery, authentication, association establishment,security setup, etc. The link setup process may also be referred to as asession initiation process or a session setup process. In addition,discovery, authentication, association, and security setup of the linksetup process may also be called an association process.

An exemplary link setup process is described with reference to FIG. 5.

In step S510, an STA may perform a network discovery action. The networkdiscovery action may include an STA scanning action. That is, in orderto access the network, the STA should search for an available network.The STA needs to identify a compatible network before participating in awireless network and the process of identifying the network present in aspecific area is referred to as scanning.

Scanning is categorized into active scanning and passive scanning

FIG. 5 exemplarily illustrates a network discovery action including anactive scanning process. An STA performing active scanning transmits aprobe request frame in order to determine which AP is present in aperipheral region while moving between channels and waits for a responseto the probe request frame. A responder transmits a probe response framein response to the probe request frame to the STA that has transmittedthe probe request frame. Here, the responder may be an STA that hasfinally transmitted a beacon frame in a BSS of the scanned channel.Since an AP transmits a beacon frame in a BSS, the AP is a responder. Inan IBSS, since STAs of the IBSS sequentially transmit the beacon frame,a responder is not the same. For example, an STA, that has transmittedthe probe request frame at channel #1 and has received the proberesponse frame at channel #1, stores BSS-related information containedin the received probe response frame, and moves to the next channel(e.g. channel #2). In the same manner, the STA may perform scanning(i.e. probe request/response transmission and reception at Channel #2).

Although not shown in FIG. 5, the scanning action may also be carriedout using passive scanning. An STA that performs passive scanning awaitsreception of a beacon frame while moving from one channel to anotherchannel. The beacon frame is one of management frames in IEEE 802.11.The beacon frame is periodically transmitted to indicate the presence ofa wireless network and allow a scanning STA to search for the wirelessnetwork and thus join the wireless network. In a BSS, an AP isconfigured to periodically transmit the beacon frame and, in an IBSS,STAs in the IBSS are configured to sequentially transmit the beaconframe. Upon receipt of the beacon frame, the scanning STA storesBSS-related information contained in the beacon frame and records beaconframe information on each channel while moving to another channel. Uponreceiving the beacon frame, the STA may store BSS-related informationcontained in the received beacon frame, move to the next channel, andperform scanning on the next channel using the same method.

Active scanning is more advantageous than passive scanning in terms ofdelay and power consumption.

After discovering the network, the STA may perform an authenticationprocess in step S520. The authentication process may be referred to as afirst authentication process in order to clearly distinguish thisprocess from the security setup process of step S540.

The authentication process includes a process in which an STA transmitsan authentication request frame to an AP and the AP transmits anauthentication response frame to the STA in response to theauthentication request frame. The authentication frame used forauthentication request/response corresponds to a management frame.

The authentication frame may include information about an authenticationalgorithm number, an authentication transaction sequence number, a statecode, a challenge text, a robust security network (RSN), a finite cyclicgroup (FCG), etc. The above-mentioned information contained in theauthentication frame may correspond to some parts of information capableof being contained in the authentication request/response frame and maybe replaced with other information or include additional information.

The STA may transmit the authentication request frame to the AP. The APmay determine whether to permit authentication for the corresponding STAbased on the information contained in the received authenticationrequest frame. The AP may provide an authentication processing result tothe STA through the authentication response frame.

After the STA has been successfully authenticated, an associationprocess may be carried out in step S530. The association processincludes a process in which the STA transmits an association requestframe to the AP and the AP transmits an association response frame tothe STA in response to the association request frame.

For example, the association request frame may include informationassociated with various capabilities, a beacon listen interval, aservice set identifier (SSID), supported rates, supported channels, anRSN, a mobility domain, supported operating classes, a trafficindication map (TIM) broadcast request, interworking service capability,etc.

For example, the association response frame may include informationassociated with various capabilities, a status code, an association ID(AID), supported rates, an enhanced distributed channel access (EDCA)parameter set, a received channel power indicator (RCPI), a receivedsignal to noise indicator (RSNI), a mobility domain, a timeout interval(association comeback time), an overlapping BSS scan parameter, a TIMbroadcast response, a quality of service (QoS) map, etc.

The above-mentioned information may correspond to some parts ofinformation capable of being contained in the associationrequest/response frame and may be replaced with other information orinclude additional information.

After the STA has been successfully associated with the network, asecurity setup process may be performed in step S540. The security setupprocess of step S540 may be referred to as an authentication processbased on robust security network association (RSNA) request/response.The authentication process of step S520 may be referred to as a firstauthentication process and the security setup process of step S540 mayalso be simply referred to as an authentication process.

The security setup process of step S540 may include a private key setupprocess through 4-way handshaking based on, for example, an extensibleauthentication protocol over LAN (EAPOL) frame. In addition, thesecurity setup process may also be performed according to other securityschemes not defined in IEEE 802.11 standards.

WLAN Evolution

To overcome limitations of communication speed in a WLAN, IEEE 802.11nhas recently been established as a communication standard. IEEE 802.11naims to increase network speed and reliability and extend wirelessnetwork coverage. More specifically, IEEE 802.11n supports a highthroughput (HT) of 540 Mbps or more. To minimize transmission errors andoptimize data rate, IEEE 802.11n is based on MIMO using a plurality ofantennas at each of a transmitter and a receiver.

With widespread supply of a WLAN and diversified applications using theWLAN, the necessity of a new WLAN system for supporting a higherprocessing rate than a data processing rate supported by IEEE 802.11nhas recently emerged. A next-generation WLAN system supporting very highthroughput (VHT) is one of IEEE 802.11 WLAN systems which have beenrecently proposed to support a data processing rate of 1 Gbps or more ina MAC service access point (SAP), as the next version (e.g. IEEE802.11ac) of an IEEE 802.11n WLAN system.

To efficiently utilize a radio frequency (RF) channel, thenext-generation WLAN system supports a multiuser (MU)-MIMO transmissionscheme in which a plurality of STAs simultaneously accesses a channel.In accordance with the MU-MIMO transmission scheme, an AP maysimultaneously transmit packets to at least one MIMO-paired STA.

In addition, support of WLAN system operations in whitespace (WS) hasbeen discussed. For example, technology for introducing the WLAN systemin TV WS such as an idle frequency band (e.g. 54 to 698 MHz band) due totransition to digital TVs from analog TVs has been discussed under theIEEE 802.11af standard. However, this is for illustrative purposes only,and the WS may be a licensed band capable of being primarily used onlyby a licensed user. The licensed user is a user who has authority to usethe licensed band and may also be referred to as a licensed device, aprimary user, an incumbent user, etc.

For example, an AP and/or STA operating in WS should provide a functionfor protecting the licensed user. As an example, assuming that thelicensed user such as a microphone has already used a specific WSchannel which is a frequency band divided by regulations so as toinclude a specific bandwidth in the WS band, the AP and/or STA cannotuse the frequency band corresponding to the corresponding WS channel inorder to protect the licensed user. In addition, the AP and/or STAshould stop using the corresponding frequency band under the conditionthat the licensed user uses a frequency band used for transmissionand/or reception of a current frame.

Therefore, the AP and/or STA needs to determine whether a specificfrequency band of a WS band can be used, in other words, whether alicensed user is present in the frequency band. A scheme for determiningwhether a licensed user is present in a specific frequency band isreferred to as spectrum sensing. An energy detection scheme, a signaturedetection scheme, etc. are used as the spectrum sensing mechanism. TheAP and/or STA may determine that the frequency band is being used by alicensed user if the intensity of a received signal exceeds apredetermined value or if a DTV preamble is detected.

Machine-to-machine (M2M) communication technology has been discussed asnext generation communication technology. Technical standard forsupporting M2M communication has been developed as IEEE 802.11ah in anIEEE 802.11 WLAN system. M2M communication refers to a communicationscheme including one or more machines or may also be called machine typecommunication (MTC) or machine-to-machine communication. In this case,the machine refers to an entity that does not require directmanipulation or intervention of a user. For example, not only a meter orvending machine including a radio communication module but also a userequipment (UE) such as a smartphone capable of performing communicationby automatically accessing a network without usermanipulation/intervention may be machines. M2M communication may includedevice-to-device (D2D) communication and communication between a deviceand an application server. As exemplary communication between a deviceand an application server, communication between a vending machine andan application server, communication between a point of sale (POS)device and an application server, and communication between an electricmeter, a gas meter, or a water meter and an application server. M2Mcommunication-based applications may include security, transportation,healthcare, etc. In the case of considering the above-mentionedapplication examples, M2M communication has to support occasionaltransmission/reception of a small amount of data at low speed under anenvironment including a large number of devices.

More specifically, M2M communication should support a large number ofSTAs. Although a currently defined WLAN system assumes that one AP isassociated with a maximum of 2007 STAs, methods for supporting othercases in which more STAs (e.g. about 6000 STAs) than 2007 STAs areassociated with one AP have been discussed in M2M communication. Inaddition, it is expected that many applications forsupporting/requesting a low transfer rate are present in M2Mcommunication. In order to smoothly support these requirements, an STAin the WLAN system may recognize the presence or absence of data to betransmitted thereto based on a TIM element and methods for reducing thebitmap size of the TIM have been discussed. In addition, it is expectedthat much traffic having a very long transmission/reception interval ispresent in M2M communication. For example, a very small amount of datasuch as electric/gas/water metering needs to be transmitted and receivedat long intervals (e.g. every month). Accordingly, although the numberof STAs associated with one AP increases in the WLAN system, methods forefficiently supporting the case in which there are a very small numberof STAs each including a data frame to be received from the AP duringone beacon period has been discussed.

As described above, WLAN technology is rapidly developing and not onlythe above-mentioned exemplary technologies but also other technologiesincluding direct link setup, improvement of media streaming throughput,support of high-speed and/or large-scale initial session setup, andsupport of extended bandwidth and operating frequency are beingdeveloped.

Medium Access Mechanism

In a WLAN system based on IEEE 802.11, a basic access mechanism ofmedium access control (MAC) is a carrier sense multiple access withcollision avoidance (CSMA/CA) mechanism. The CSMA/CA mechanism is alsoreferred to as a distributed coordination function (DCF) of the IEEE802.11 MAC and basically adopts a “listen before talk” access mechanism.In this type of access mechanism, an AP and/or an STA may sense awireless channel or a medium during a predetermined time duration (e.g.DCF interframe space (DIFS) before starting transmission. As a result ofsensing, if it is determined that the medium is in an idle status, theAP and/or the STA starts frame transmission using the medium. Meanwhile,if it is sensed that the medium is in an occupied state, the AP and/orthe STA does not start its transmission and may attempt to perform frametransmission after setting and waiting for a delay duration (e.g. arandom backoff period) for medium access. Since it is expected thatmultiple STAs attempt to perform frame transmission after waiting fordifferent time durations by applying the random backoff period,collision can be minimized.

An IEEE 802.11 MAC protocol provides a hybrid coordination function(HCF) based on the DCF and a point coordination function (PCF). The PCFrefers to a scheme of performing periodic polling by using apolling-based synchronous access method so that all reception APs and/orSTAs can receive a data frame. The HCF includes enhanced distributedchannel access (EDCA) and HCF controlled channel access (HCCA). EDCA isa contention based access scheme used by a provider to provide a dataframe to a plurality of users. HCCA uses a contention-free based channelaccess scheme employing a polling mechanism. The HCF includes a mediumaccess mechanism for improving QoS of a WLAN and QoS data may betransmitted in both a contention period (CP) and a contention-freeperiod (CFP).

FIG. 6 is a diagram for explaining a backoff process.

Operations based on a random backoff period will now be described withreference to FIG. 6. If a medium of an occupy or busy state transitionsto an idle state, several STAs may attempt to transmit data (or frames).As a method for minimizing collision, each STA may select a randombackoff count, wait for a slot time corresponding to the selectedbackoff count, and then attempt to start data or frame transmission. Therandom backoff count may be a pseudo-random integer and may be set toone of 0 to CW values. In this case, CW is a contention window parametervalue. Although CWmin is given as an initial value of the CW parameter,the initial value may be doubled in case of transmission failure (e.g.in the case in which ACK for the transmission frame is not received). Ifthe CW parameter value reaches CWmax, the STAs may attempt to performdata transmission while CWmax is maintained until data transmission issuccessful. If data has been successfully transmitted, the CW parametervalue is reset to CWmin. Desirably, CW, CWmin, and CWmax are set to 2n−1(where n=0, 1, 2, . . . ).

If the random backoff process is started, the STA continuously monitorsthe medium while counting down the backoff slot in response to thedetermined backoff count value. If the medium is monitored as theoccupied state, the countdown stops and waits for a predetermined time.If the medium is in the idle status, the remaining countdown restarts.

As shown in the example of FIG. 6, if a packet to be transmitted to MACof STA3 arrives at STA3, STA3 may confirm that the medium is in the idlestate during a DIFS and directly start frame transmission. In themeantime, the remaining STAs monitor whether the medium is in the busystate and wait for a predetermined time. During the predetermined time,data to be transmitted may occur in each of STA1, STA2, and STA5. If itis monitored that the medium is in the idle state, each STA waits forthe DIFS time and then may perform countdown of the backoff slot inresponse to a random backoff count value selected by each STA. Theexample of FIG. 6 shows that STA2 selects the lowest backoff count valueand STA1 selects the highest backoff count value. That is, after STA2finishes backoff counting, the residual backoff time of STA5 at a frametransmission start time is shorter than the residual backoff time ofSTA1. Each of STA1 and STA5 temporarily stops countdown while STA2occupies the medium, and waits for a predetermined time. If occupationof STA2 is finished and the medium re-enters the idle state, each ofSTA1 and STA5 waits for a predetermined time DIFS and restarts backoffcounting. That is, after counting down the remaining backoff timecorresponding to the residual backoff time, each of STA1 and STA5 maystart frame transmission. Since the residual backoff time of STA5 isshorter than that of STA1, STA5 starts frame transmission. Meanwhile,data to be transmitted may occur even in STA4 while STA2 occupies themedium. In this case, if the medium is in the idle state, STA4 may waitfor the DIFS time, perform countdown in response to the random backoffcount value selected thereby, and then start frame transmission. FIG. 6exemplarily shows the case in which the residual backoff time of STA5 isidentical to the random backoff count value of STA4 by chance. In thiscase, collision may occur between STA4 and STA5. Then, each of STA4 andSTA5 does not receive ACK, resulting in occurrence of data transmissionfailure. In this case, each of STA4 and STA5 may increase the CW valueby two times, select a random backoff count value, and then performcountdown. Meanwhile, STA1 waits for a predetermined time while themedium is in the occupied state due to transmission of STA4 and STA5. Ifthe medium is in the idle state, STA1 may wait for the DIFS time andthen start frame transmission after lapse of the residual backoff time.

STA Sensing Operation

As described above, the CSMA/CA mechanism includes not only a physicalcarrier sensing mechanism in which the AP and/or an STA directly sensesa medium but also a virtual carrier sensing mechanism. The virtualcarrier sensing mechanism can solve some problems such as a hidden nodeproblem encountered in medium access. For virtual carrier sensing, MACof the WLAN system may use a network allocation vector (NAV). The NAV isa value used to indicate a time remaining until an AP and/or an STAwhich is currently using the medium or has authority to use the mediumenters an available state to another AP and/or STA. Accordingly, a valueset to the NAV corresponds to a reserved time in which the medium willbe used by an AP and/or STA configured to transmit a correspondingframe. An STA receiving the NAV value is not allowed to perform mediumaccess during the corresponding reserved time. For example, NAV may beset according to the value of a ‘duration’ field of a MAC header of aframe.

A robust collision detection mechanism has been proposed to reduce theprobability of collision. This will be described with reference to FIGS.7 and 8. Although an actual carrier sensing range is different from atransmission range, it is assumed that the actual carrier sensing rangeis identical to the transmission range for convenience of description.

FIG. 7 is a diagram for explaining a hidden node and an exposed node.

FIG. 7(a) exemplarily shows a hidden node. In FIG. 7(a), STA Acommunicates with STA B, and STA C has information to be transmitted.Specifically, STA C may determine that a medium is in an idle state whenperforming carrier sensing before transmitting data to STA B, althoughSTA A is transmitting information to STA B. This is because transmissionof STA A (i.e. occupation of the medium) may not be detected at thelocation of STA C. In this case, STA B simultaneously receivesinformation of STA A and information of STA C, resulting in occurrenceof collision. Here, STA A may be considered a hidden node of STA C.

FIG. 7(b) exemplarily shows an exposed node. In FIG. 7(b), in asituation in which STA B transmits data to STA A, STA C has informationto be transmitted to STA D. If STA C performs carrier sensing, it isdetermined that a medium is occupied due to transmission of STA B.Therefore, although STA C has information to be transmitted to STA D,since the medium-occupied state is sensed, STA C should wait for apredetermined time until the medium is in the idle state. However, sinceSTA A is actually located out of the transmission range of STA C,transmission from STA C may not collide with transmission from STA Bfrom the viewpoint of STA A, so that STA C unnecessarily enters astandby state until STA B stops transmission. Here, STA C is referred toas an exposed node of STA B.

FIG. 8 is a diagram for explaining request to send (RTS) and clear tosend (CTS).

To efficiently utilize a collision avoidance mechanism under theabove-mentioned situation of FIG. 7, it is possible to use a shortsignaling packet such as RTS and CTS. RTS/CTS between two STAs may beoverheard by peripheral STA(s), so that the peripheral STA(s) mayconsider whether information is transmitted between the two STAs. Forexample, if an STA to be used for data transmission transmits an RTSframe to an STA receiving data, the STA receiving data may informperipheral STAs that itself will receive data by transmitting a CTSframe to the peripheral STAs.

FIG. 8(a) exemplarily shows a method for solving problems of a hiddennode. In FIG. 8(a), it is assumed that both STA A and STA C are ready totransmit data to STA B. If STA A transmits RTS to STA B, STA B transmitsCTS to each of STA A and STA C located in the vicinity of the STA B. Asa result, STA C waits for a predetermined time until STA A and STA Bstop data transmission, thereby avoiding collision.

FIG. 8(b) exemplarily shows a method for solving problems of an exposednode. STA C performs overhearing of RTS/CTS transmission between STA Aand STA B, so that STA C may determine that no collision will occuralthough STA C transmits data to another STA (e.g. STA D). That is, STAB transmits RTS to all peripheral STAs and only STA A having data to beactually transmitted may transmit CTS. STA C receives only the RTS anddoes not receive the CTS of STA A, so that it can be recognized that STAA is located outside of the carrier sensing range of STA C.

Power Management

As described above, the WLAN system needs to perform channel sensingbefore an STA performs data transmission/reception. The operation ofalways sensing the channel causes persistent power consumption of theSTA. Power consumption in a reception state is not greatly differentfrom that in a transmission state. Continuous maintenance of thereception state may cause large load to a power-limited STA (i.e. an STAoperated by a battery). Therefore, if an STA maintains a receptionstandby mode so as to persistently sense a channel, power isinefficiently consumed without special advantages in terms of WLANthroughput. In order to solve the above-mentioned problem, the WLANsystem supports a power management (PM) mode of the STA.

The PM mode of the STA is classified into an active mode and a powersave (PS) mode. The STA basically operates in the active mode. The STAoperating in the active mode maintains an awake state. In the awakestate, the STA may perform a normal operation such as frametransmission/reception or channel scanning On the other hand, the STAoperating in the PS mode is configured to switch between a sleep stateand an awake state. In the sleep state, the STA operates with minimumpower and performs neither frame transmission/reception nor channelscanning.

Since power consumption is reduced in proportion to a specific time inwhich the STA stays in the sleep state, an operation time of the STA isincreased. However, it is impossible to transmit or receive a frame inthe sleep state so that the STA cannot always operate for a long periodof time. If there is a frame to be transmitted to an AP, the STAoperating in the sleep state is switched to the awake state totransmit/receive the frame. On the other hand, if the AP has a frame tobe transmitted to the STA, the sleep-state STA is unable to receive theframe and cannot recognize the presence of a frame to be received.Accordingly, the STA may need to switch to the awake state according toa specific period in order to recognize the presence or absence of aframe to be transmitted thereto (or in order to receive the frame if theAP has the frame to be transmitted thereto).

FIG. 9 is a diagram for explaining a PM operation.

Referring to FIG. 9, an AP 210 transmits a beacon frame to STAs presentin a BSS at intervals of a predetermined time period (S211, S212, S213,S214, S215, and S216). The beacon frame includes a TIM informationelement. The TIM information element includes buffered traffic regardingSTAs associated with the AP 210 and includes information indicating thata frame is to be transmitted. The TIM information element includes a TIMfor indicating a unicast frame and a delivery traffic indication map(DTIM) for indicating a multicast or broadcast frame.

The AP 210 may transmit a DTIM once whenever the beacon frame istransmitted three times. Each of STA1 220 and STA2 222 operate in a PSmode. Each of STA1 220 and STA2 222 is switched from a sleep state to anawake state every wakeup interval of a predetermined period such thatSTA1 220 and STA2 222 may be configured to receive the TIM informationelement transmitted by the AP 210. Each STA may calculate a switchingstart time at which each STA may start switching to the awake statebased on its own local clock. In FIG. 9, it is assumed that a clock ofthe STA is identical to a clock of the AP.

For example, the predetermined wakeup interval may be configured in sucha manner that STA1 220 can switch to the awake state to receive the TIMelement every beacon interval. Accordingly, STA1 220 may switch to theawake state when the AP 210 first transmits the beacon frame (S211).STA1 220 may receive the beacon frame and obtain the TIM informationelement. If the obtained TIM element indicates the presence of a frameto be transmitted to STA1 220, STA1 220 may transmit a power save-Poll(PS-Poll) frame, which requests the AP 210 to transmit the frame, to theAP 210 (S221 a). The AP 210 may transmit the frame to STA1 220 inresponse to the PS-Poll frame (S231). STA1 220 which has received theframe is re-switched to the sleep state and operates in the sleep state.

When the AP 210 secondly transmits the beacon frame, since a busy mediumstate in which the medium is accessed by another device is obtained, theAP 210 may not transmit the beacon frame at an accurate beacon intervaland may transmit the beacon frame at a delayed time (S212). In thiscase, although STA1 220 is switched to the awake state in response tothe beacon interval, STA1 does not receive the delay-transmitted beaconframe so that it re-enters the sleep state (S222).

When the AP 210 thirdly transmits the beacon frame, the correspondingbeacon frame may include a TIM element configured as a DTIM. However,since the busy medium state is given, the AP 210 transmits the beaconframe at a delayed time (S213). STA1 220 is switched to the awake statein response to the beacon interval and may obtain a DTIM through thebeacon frame transmitted by the AP 210. It is assumed that the DTIMobtained by STA1 220 does not have a frame to be transmitted to STA1 220and there is a frame for another STA. In this case, STA1 220 may confirmthe absence of a frame to be received in the STA1 220 and re-enters thesleep state so that the STA1 220 may operate in the sleep state. Aftertransmitting the beacon frame, the AP 210 transmits the frame to thecorresponding STA (S232).

The AP 210 fourthly transmits the beacon frame (S214). However, since itwas impossible for STA1 220 to obtain information regarding the presenceof buffered traffic associated therewith through previous doublereception of a TIM element, STA1 220 may adjust the wakeup interval forreceiving the TIM element. Alternatively, provided that signalinginformation for coordination of the wakeup interval value of STA1 220 iscontained in the beacon frame transmitted by the AP 210, the wakeupinterval value of the STA1 220 may be adjusted. In this example, STA1220, which has been switched to receive a TIM element every beaconinterval, may be configured to be switched to another operation state inwhich STA1 220 awakes from the sleep state once every three beaconintervals. Therefore, when the AP 210 transmits a fourth beacon frame(S214) and transmits a fifth beacon frame (S215), STA1 220 maintains thesleep state such that it cannot obtain the corresponding TIM element.

When the AP 210 sixthly transmits the beacon frame (S216), STA1 220 isswitched to the awake state and operates in the awake state, so that theSTA1 220 may obtain the TIM element contained in the beacon frame(S224). The TIM element is a DTIM indicating the presence of a broadcastframe. Accordingly, STA1 220 does not transmit the PS-Poll frame to theAP 210 and may receive the broadcast frame transmitted by the AP 210(S234). In the meantime, the wakeup interval configured for STA2 230 maybe longer than the wakeup interval of STA1 220. Accordingly, STA2 230may enter the awake state at a specific time (S215) where the AP 210fifthly transmits the beacon frame and receives the TIM element (S241).STA2 230 may recognize the presence of a frame to be transmitted theretothrough the TIM element and transmit the PS-Poll frame to the AP 210 torequest frame transmission (S241 a). The AP 210 may transmit the frameto STA2 230 in response to the PS-Poll frame (S233).

In order to manage a PS mode shown in FIG. 9, the TIM element mayinclude either a TIM indicating the presence or absence of a frame to betransmitted to the STA or include a DTIM indicating the presence orabsence of a broadcast/multicast frame. The DTIM may be implementedthrough field setting of the TIM element.

FIGS. 10 to 12 are diagrams for explaining detailed operations of an STAthat has received a TIM.

Referring to FIG. 10, an STA is switched from a sleep state to an awakestate so as to receive a beacon frame including a TIM from an AP. TheSTA may recognize the presence of buffered traffic to be transmittedthereto by interpreting the received TIM element. After contending withother STAs to access a medium for PS-Poll frame transmission, the STAmay transmit the PS-Poll frame for requesting data frame transmission tothe AP. Upon receiving the PS-Poll frame transmitted by the STA, the APmay transmit the frame to the STA. The STA may receive a data frame andthen transmit an ACK frame to the AP in response to the received dataframe. Thereafter, the STA may re-enter the sleep state.

As illustrated in FIG. 10, the AP may operate according to an immediateresponse scheme in which the AP receives the PS-Poll frame from the STAand transmits the data frame after a predetermined time (e.g. a shortinterframe space (SIFS)). Meanwhile, if the AP does not prepare a dataframe to be transmitted to the STA during the SIFS time after receivingthe PS-Poll frame, the AP may operate according to a deferred responsescheme and this will be described with reference to FIG. 11.

The STA operations of FIG. 11 in which an STA is switched from a sleepstate to an awake state, receives a TIM from an AP, and transmits aPS-Poll frame to the AP through contention are identical to those ofFIG. 10. Even upon receiving the PS-Poll frame, if the AP does notprepare a data frame during an SIFS time, the AP may transmit an ACKframe to the STA instead of transmitting the data frame. If the dataframe is prepared after transmission of the ACK frame, the AP maytransmit the data frame to the STA after completion of contention. TheSTA may transmit the ACK frame indicating that the data frame hassuccessfully been received to the AP and transition to the sleep state.

FIG. 12 illustrates an exemplary case in which an AP transmits a DTIM.STAs may be switched from the sleep state to the awake state so as toreceive a beacon frame including a DTIM element from the AP. The STAsmay recognize that a multicast/broadcast frame will be transmittedthrough the received DTIM. After transmission of the beacon frameincluding the DTIM, the AP may directly transmit data (i.e. themulticast/broadcast frame) without transmitting/receiving a PS-Pollframe. While the STAs continuously maintains the awake state afterreception of the beacon frame including the DTIM, the STAs may receivedata and then switch to the sleep state after completion of datareception.

TIM Structure

In case of a method of managing a power saving mode based on the TIM(DTIM) protocol mentioned earlier with reference to FIGS. 9 to 12, STAscan check whether or not there exists a data frame to be transmitted tothe STAs via STA identification information included in a TIM element.The STA identification information may correspond to information relatedto an AID (association identifier) which is an identifier assigned to anSTA when the STA is associated with an AP.

The AID is used as a unique identifier for each STA in a single BSS. Asan example, the AID is assigned by a value among values ranging from 1to 2007 in a current wireless LAN system. In a currently definedwireless LAN system, 14 bits can be assigned to a frame transmitted byan AP and/or an STA as the AID. Although a value of the AID can beassigned up to 16383, values ranging from 2008 to 16383 are configuredas reserved values.

A TIM element according to a legacy definition is not suitable for beingapplied to an M2M application that many numbers (e.g., over 2007) ofSTAs are associated with a single AP. In case of expanding a legacy TIMstructure as it is, since a size of a TIM bitmap becomes too large, itis unable to support with a legacy frame format and it is notappropriate for M2M communication considering an application of a lowtransmission rate. And. It is expected that the number of STAs in whicha reception data frame exists during a single beacon interval is verysmall in M2M communication. Hence, in case of considering theaforementioned M2M communication application example, although a size ofa TIM bitmap is enlarged, it is expected a case that most of bits has avalue of 0 frequently occurs. Thus, a technology of efficientlycompressing a bitmap is required.

As a legacy bitmap compression technology, there is a method of omittingcontiguous 0's at the forepart of a bitmap and defining by an offset (orstart point) value. Yet, if the number of STAs in which a buffered frameexists is less and a difference of an AID value of each STA is big, acompression efficiency of the method is not high. For example, when aframe, which is to be transmitted to 2 STAs respectively including anAID of 10 and an AID of 2000, is buffered only, although a length of acompressed bitmap corresponds to 1990, all bits have a value of 0 exceptboth ends. If the number of STAs capable of being associated with asingle AP is less, inefficiency of bitmap compression is not a bigproblem. Yet, if the number of STAs increases, the inefficiency maybecome an element deteriorating overall system performance

As a method of solving the aforementioned problem, data transmission canbe more efficiently performed in a manner of dividing an AID into aplurality of groups. A designated group ID (GID) is assigned to each ofa plurality of the groups. The AID assigned based on a group isexplained with reference to FIG. 13 in the following.

Several bits at the front of an AID bitmap can be used to indicate aGID. For example, first 2 bits of the AID bitmap can be used forindicating 4 GIDs. When the total length of an AID bitmap corresponds toN bits, a value of first 2 bits (B1 and B2) indicates a GID of the AID.

A GID can be assigned according to a position of an AID. In this case,AIDs using an identical GID can be represented by a value of an offsetand a length. For example, if a GID 1 is represented by an offset A anda length B, it means that AIDs ranging from A to A+B−1 have the GID 1 ona bitmap. For example, assume that the total AIDs ranging from 1 to N4are divided into 4 groups. In this case, AIDs belonging to the GID 1correspond to AIDs ranging from 1 to N1 and the AIDs belonging to theGID 1 can be represented by an offset 1 and a length N1. AIDs belongingto a GID 2 can be represented by an offset N1+1 and a length N2−N1+1,AIDs belonging to a GID 3 can be represented by an offset N2+1 and alength N3−N2+1 and AIDs belonging to a GID 4 can be represented by anoffset N3+1 and a length N4−N3+1.

As mentioned in the foregoing description, if an AID assigned based on agroup is introduced, it is able to make channel access to be permittedin time section different from each other according to a GID. Hence, aTIM element deficiency problem for many numbers of STAs is solved anddata can be efficiently transmitted and received. For example, channelaccess is permitted for STA(s) belonging to a specific group only duringspecific time section and the rest of STA(s) may have restriction on thechannel access. A prescribed time section for which access is permittedfor specific STA(s) may be called a RAW (restricted access window).

With respect to a channel access according to a GID, a channel accessmechanism according to a beacon interval when an AID is divided into 3groups is described below. A first beacon interval (first RAW)corresponds to an interval for which a channel access of an STAcorresponding to an AID belonging to a GID 1 is permitted Channel accessof STAs belonging to a different GID is not permitted. To this end, ATIM element for AIDs corresponding to the GID 1 is included in the firstbeacon only. A TIM element for AIDs including a GID 2 is included in asecond beacon frame. Hence, channel access of STAs corresponding to AIDsbelonging to the GID 2 is permitted only during a second beacon interval(second RAW). A TIM element for AIDs including a GID 3 is included in athird beacon interval only. Hence, channel access of STAs correspondingto AIDs belonging to the GID 3 is permitted only during a third beaconinterval (third RAW). The TIM element for the AIDs including the GID 1is included again in a fourth beacon interval only. Hence, channelaccess of the STAs corresponding to the AIDs belonging to the GID 1 ispermitted only during a fourth beacon interval (fourth RAW). Channelaccess of an STA belonging to a specific group, which is indicated by aTIM included in a corresponding beacon frame, is permitted only duringeach of beacon intervals after a fifth beacon interval (each of RAWsafter a fifth RAW).

In the above example, the order of the GIDs permitted according to thebeacon interval is shown to be cyclic or periodic, but the presentinvention is not limited thereto. In particular, if AID(s) belonging toa specific GID(s) is included in a TIM element, channel access of STA(s)corresponding to the specific AID(s) can be permitted during specifictime interval (specific RAW) and channel access of the rest of STA(s)may not be permitted during the specific time interval.

As mentioned in the foregoing description, the group-based AIDassignment scheme can also be called a hierarchical structure of a TIM.In particular, a total AID space is divided into a plurality of blocksand it is able to make channel access of STA(s) (i.e., STA of a specificgroup) corresponding to a specific block including a value except 0 tobe permitted only. By doing so, a TIM of a large size is divided into asmall blocks/groups, an STA can easily maintain TIM information and theblocks/groups can be easily managed according to a class of an STA,service quality (QoS), or a usage.

In the examples of the present invention described in the following, itis able to apply various methods of dividing STAs (or AIDs assigned toeach of the STAs) in a prescribed hierarchical group unit and managingthe STAs. A group-based AID assignment scheme may be non-limited by theexamples.

Examples of Frame Format

FIG. 13 is a diagram for an example of an MAC frame format of IEEE802.11 system to which the present invention is applicable.

Referring to FIG. 13, a MAC frame format includes a MAC header (MHR), aMAC payload and a MAC footer (MFR). The MHR is defined by a regionincluding a frame control field, a duration/ID field, an address 1field, an address 2 field, an address 3 field, a sequence control field,an address 4 field, a QoS control field and a HT control field. A framebody field is defined by the MAC payload. Data intended to betransmitted by upper layer is positioned at the frame body field. Theframe body field has a variable size. A frame check sequence (FCS) fieldis defined by the MAC footer and is used to detect an error of the MACframe.

A minimum frame format is configured by the first three fields (theframe control field, the duration/ID field and the address 1 field) anda very last field (the FCS field). The first three fields and the lastfield exist in all frames. The remaining fields can exist in a specificframe type only.

Information included in each of the aforementioned fields may follow thedefinition of IEEE 802.11 system. And, the each of the aforementionedfields corresponds to an example of fields capable of being included ina MAC frame. Each field can be replaced with a different field or anadditional field can be further included as well.

FIG. 14 is a diagram for an example of an HT format of an HT controlfield in a MAC frame according to FIG. 13.

Referring to FIG. 14, the HT control field can include a VHT subfield, alink adaptation subfield, a calibration position subfield, a calibrationsequence subfield, a channel state information(CSI)/steering subfield,an NDP (null data packet) announcement subfield, an AC (access category)constraint subfield, an RDG (reverse direction grant/more) PPDU subfieldand a reserved subfield.

The link adaptation subfield can include a training request (TRQ)subfield, an MAI (MCS (modulation and coding scheme) request or an ASEL(antenna selection) indication) subfield, an MCS feedback sequenceindication (MFSI) subfield, an MCS feedback and antenna selectioncommand/data (MFB/ASELC) subfield.

If a sounding PPDU is requested to a responder, the TRQ subfield is setto 1. If the sounding PPDU is not requested to the responder, the TRQsubfield is set to 0. And, if the MAI subfield is set to 14, itindicates an antenna selection indication (ASEL indication) and theMFB/ASELC subfield is interpreted by the antenna selection command/data.Otherwise, the MAI subfield indicates an MCS request and the MFB/ASELCsubfield is interpreted by an MCS feedback. When the MAI subfieldindicates an MCS request (MRO), if MCS feedback is not requested, theMAI subfield is set to 0. If the MCS is requested, the MAI subfield isset to 1. The sounding PPDU indicates a PPDU delivering a trainingsymbol usable for channel estimation.

The aforementioned each of the subfields corresponds to an example ofsubfields capable of being included in the HT control field. Each fieldcan be replaced with a different subfield. Or, an additional subfieldcan be further included.

FIG. 15 is a diagram for an example of a VHT format of an HT controlfield in a MAC frame according to FIG. 13.

Referring to FIG. 15, the HT control field can include a VHT subfield,an MRO subfield, an MSI subfield, an MCS feedback sequenceindication/group ID lowest bit (MFSI/GID-L: LSB of group ID) subfield,an MFB subfield, a group ID highest bit (GID-H: MSB of group ID)subfield, a coding type subfield, an MFC response transmission type (FBTx type: transmission type of MFB response) subfield, an unsolicited MFBsubfield, an AC constraint subfield, an RDG/more PPDU subfield. And, theMFB subfield can include a VHT space-time stream number (N_STS: numberof space time streams) subfield, an MCS subfield, a bandwidth (BW)subfield and a signal to noise ratio (SNR) subfield.

Table 1 shows explanation on each subfield in a VHT format of the HTcontrol field.

TABLE 1 Subfield Meaning Definition MRQ MCS request If MCS feedback(solicited MFB) is requested, set to 1. Otherwise, set to 0. MSI MRO IfMRO subfield is set to 1, MSI sequence subfield includes sequence numberidentifier within a scope ranging from 0 to 6 identifying a specificrequest. If MRO subfield is set to 0, MSI subfield is reserved. MFSI/MFB If unsolicited MFB subfield is set GID-L sequence to 0, MFSI/GID-Lsubfield includes identifier/ a reception value of MSI included LSB ofin a frame indicated by MFB group ID information. If unsolicited MFBsubfield is set to 1, MFSI/GID-L subfield includes lowest 3 bits of agroup ID of PPDU indicated by solicited MFB. MFB VHT N_STS, MFB subfieldincludes a recommended MCS, BW, MFB. MCS = 15, VHT N_STS = 7 SNRindicate that there is no feedback. feedback GID-H MSB of If unsolicitedMFB subfield is set group ID to 1, GID-H subfield includes highest 3bits of a group ID of PPDU indicated by the unsolicited MFB. CodingCoding If unsolicited MFB subfield is set type type to 1, coding typesubfield includes of MFB 1 in case of coding information response (BCC(binary convolution code)) indicated by the unsolicited MFB, 0 in caseof LDPC (low-density parity check). Otherwise, reserved. FB TxTransmission If unsolicited MFB subfield is set type type of MFB to 1and FB Tx type subfield is response set to 0, the unsolicited MFBindicates either unbeamformed VHT PPDU or transmit diversity using STBC(space-time block coding) VHT PPDU. If unsolicited MFB subfield is setto 1 and FB Tx type subfield is set to 1, the unsolicited MFB indicatesbeamformed SU-MIMO (single user MIMO) VHT PPDU. Otherwise, reserved.Unso- Unsolicited If MFB is not a response of MRQ, licited MCS feedbackset to 1. If MFB is a response of MFB indicator MRQ, set to 0. Ac Ifresponse for reverse direction constraint grant (RDG) includes dataframe from a traffic identifier (TID), set to 0. If response for reversedirection grant (RDG) includes a frame received from AC identical tolast data frame received from an identical reverse direction (RD)initiator only, set to 1. RDG/more When RDG/more PPDU subfield PPDUcorresponds to 0, if reverse direction (RD) initiator transmits, itindicates there is no reverse direction grant (RDG). If reversedirection (RD) responder transmits, it indicates PPDU delivering MACframe is last transmission. When RDG/more PPDU subfield corresponds to1, if reverse direction (RD) initiator transmits, it indicates thereexists reverse direction grant (RDG). If reverse direction (RD)responder transmits, there exist following different PPDU after PPDUdelivering MAC frame.

The aforementioned each of the subfields corresponds to an example ofsubfields capable of being included in the HT control field. Each fieldcan be replaced with a different subfield. Or, an additional subfieldcan be further included.

In the meantime, the MAC sub-layer delivers an MAC protocol data unit(MPDU) to a physical layer as a physical service data unit (PSDU). APCCP entity adds a physical header and a preamble to the received PSDUand generates a PLCP protocol data unit (PPDU).

FIG. 16 is a diagram for an example of a PPDU frame format of IEEE802.11n system to which the present invention is applicable.

FIG. 16(a) shows an example of a PPDU frame according to a non-HTformat, an HT mixed format and an HT-greenfield format.

The non-HT format indicates a frame format for a legacy system (IEEE802.11 a/g) STA. A non-HT format PPDU includes a legacy format preambleconsisting of a legacy-short training field (L-STF), a legacy-longtraining field (L-LTF) and a legacy-signal (L-SIG) field.

The HT mixed format permits a communication with a legacy system STA andindicates a frame format for IEEE 802.11n STA at the same time. The HTmixed format PPDU includes a legacy format preamble consisting of theL-STF, the L-LTF and the L-SIG and an HT format preamble consisting ofan HT-short training field (HT-STF), an HT-long training field (HT-LTF)and an HT-signal (HT-SIG) field. Since the L-STF, the L-LTF and theL-SIG mean legacy fields for backward compatibility, a part from theL-STF to the L-SIG is identical to the non-HT format. An STA canidentify the mixed format PPDU using the HT-SIG field appearing afterthe part.

The HT-greenfield format is a format not compatible with a legacysystem. The HT-greenfield format indicates a format used for an IEEE802.11n STA. an HT-greenfield format PPDU includes a greenfield preambleconsisting of an HT-greenfield-STF (HT-GF-STF), an HT-LTF1, an HT-SIGand one or more HT-LTFs.

A data field includes a service field, PSDU, tail bit and pad bit. Allbits of the data field are scrambled.

FIG. 16(b) shows the service field included in the data field. Theservice field has 16 bits. Each bit is numbered by 0 to 15. Each bit issequentially transmitted from a bit #0. The bit #0 to a bit #6 are setto 0 and used to synchronize a descrambler installed in a receiving end.

FIG. 17 is a diagram for an example of a VHT PPDU frame format of IEEE802.11ac system to which the present invention is applicable.

Referring to FIG. 17, a VHT format PPDU includes a legacy formatpreamble consisting of L-STF, L-LTF and L-SIG and a VHT format preambleconsisting of VHT-SIG-A, HT-STF and HT-LTF before a data field. Sincethe L-STF, the L-LTF and the L-SIG mean a legacy field for backwardcompatibility, a part from the L-STF to the L-SIG is identical to thenon-HT format. An STA can identify the VHT format PPDU using the VHT-SIGfield appearing after the part.

The L-STF is a field used for frame detection, auto gain control (AGC)diversity detection, coarse frequency/time synchronization, and thelike. The L-LTF is a field used for fine frequency/time synchronization,channel estimation, and the like. The L-SIG is a field used fortransmitting legacy control information. The VHT-SIG-A is a VHT fieldused for transmitting control information included in VHT STAs incommon. The VHT-STF is a field used for AGC for MIMO and a beamformedstream. The VHT-LTFs is a field used for channel estimation for MIMO anda beamformed stream. The VHT-SIG-B is a field used for transmittingcontrol information specific to each STA. Structures of the VHT-SIG-Aand the VHT-SIG-B are shown in FIG. 18(a) and FIG. 18(b), respectively.

FIG. 19 is a diagram for an example of a PPDU format in 11ax.

Referring to the example of FIG. 19(a), HE-SIG1 appears right afterL-part (L-STF, L-LTF, L-SIG). Similar to the L-part, the HE-SIG1 isduplicated in a unit of 20 MHz The HE-SIG1 includes common information(BW, GI length, BSS index, CRC, Tail, etc.). Referring to the structureof FIG. 19(b), the HE-SIG1 includes user allocation information (e.g.,STA's ID (PAID or GID), allocated resource information, Nsts, etc.).HE-SIG2 is transmitted per OFDMA allocation. In case of performingMU-MIMO, the HE-SIG2 is identified by an STA via SDM. The HE-SIG2includes additional user allocation information (e.g., MCS, coding,STBC, TXBE, etc.). Referring to FIG. 19(c), the HE-SIG2 is transmittedimmediately after the HE-SIG1 via information (numerology) of theHE-SIG1 over the full band. The HE-SIG2 includes user allocationinformation (e.g., STA AID, resource allocation information (e.g.,allocation size), MCS, Nsts, coding, STBC, TXBF, etc.).

FIG. 20 is a diagram for an example of 11ax MU PS poll. An AP transmitsa TIM beacon frame. The TIM beacon frame can include informationnecessary for STAs performing MU transmission to receive a trigger frame(e.g., resource allocation, start offset, trigger frame transmissiontiming, etc.). If an STA receives a trigger frame from the AP, the STAcan perform the MU transmission after prescribed time (e.g., SIFS). TheAP can transmit DL MU data or ACK to the MU STA after SIFS.

FIG. 21 is a diagram for an example of a UL MU U-APSD operation. If STAsreceive TIM from an AP, the STAs obtain transmission information of atrigger frame, receive the trigger frame, and may be able to transmit aUL MU QoS null frame to the AP. Having received the UL MU QoS nullframe, the AP can transmit DL data after SIFS. In this case, thetransmission information of the trigger frame can be informed in amanner that TIM indicates a start offset of a trigger frame to bereceived by each STA (method 1) or each trigger frame indicates a startoffset for a different STA (method 2).

FIG. 22 is a diagram for an example of transmitting a plurality oftrigger frames during a single beacon interval. In this case, a TIMbeacon frame can transmit start off information for each of a pluralityof the trigger frames.

In the following, various methods for an STA to transmit a UL frame(PS-poll, ACK/block ACK, resource request/buffer status report, CTS,NDP, etc.) are explained based on the aforementioned description. In thefollowing description, a trigger frame corresponds to a frame thattriggers UL data or short management/control frames. The UL framecorresponds to UL (MU) data or short management/control frames (PS-Poll,Ack/Block Ack, Resource Request/Buffer Status report, CTS, NDP frame,etc.). And, the STA corresponds to an STA or a plurality of STAs. TheSTA may correspond to a PS/U-APSD STA. And, the STA can support UL MU Txusing UL MU-MIMO and UL OFDMA.

Method of Performing UL Transmission after Trigger Frame is Received

According to one embodiment of the present invention, an STA receives atrigger frame from an AP and can perform uplink transmission in responseto the trigger frame. If an NAV (network allocation vector) of the STAis idle and one of a resource size allocated by the trigger frame and adata size of the uplink transmission is smaller than a thresholdindicated by the trigger frame, the STA can perform the uplinktransmission irrespective of a CCA result before the trigger frame isreceived. In particular, among STAs which have received the triggerframe, an STA including an idle NAV can perform UL frame transmissionafter SIFS immediately after the trigger frame is received irrespectiveof a CCA value of a specific duration (e.g., PIFS) before the triggerframe is received. This is because, since the AP transmits the triggerframe on an idle channel and the STA receives the trigger frame, if theSTA performs UL transmission after the trigger frame is received, it ishighly probable that the AP succeeds in reception. In case of a shortmanagement/control frame, since transmission time is short, it may lessinfluence on a different AP or an STA. If a trigger frame triggers ashort management/control frame (PS-poll, Ack/Block Ack, resourcerequest/buffer status report, CTS, NDP, etc.), an STA can transmit theshort management/control frame after SIFS irrespective of an alreadyperformed CCA result (although busy), thereby increasing transmissionefficiency.

In this case, the NAV is idle when an NAV count of the STA correspondsto 0 or a non-bandwidth signaling TA is identical to an address of aTXOP holder. The uplink transmission can be performed when SIFS elapsedafter the trigger frame is received.

A threshold value indicated by the trigger frame can be transmitted viaa beacon for configuring CCA or dynamic CCA or a management frame. Or,the AP can transmit a specific threshold value to the STA using reservedbits of a service field of a data field of HE-SIG1 and HE-SIG2.

In relation to a method of indicating a threshold value, it may be ableto use at least one of the following methods.

First of all, a threshold value indicated by a trigger frame correspondsto a decimal value *4 corresponding to bits transmitted via the triggerframe and a unit of the threshold value may correspond to Octet. Aspecific threshold value can be configured in a unit of a value of bitstransmitted by the AP*y octets (in this case, y is a random positivenumber). For example, when a specific threshold value is defined in aunit of 4 octets, if the AP transmits ‘0111’, the threshold value maycorrespond to 28 octets. In this case, if a size/length of a resource(e.g., PPDU or A-MPDU) allocated for an STA or a size/length of a ULframe (e.g., PPDU or A-MPDU) to be transmitted by an STA is equal to orless than 28 octets, it may be able to transmit a UL frame irrespectiveof a CCA value of a specific duration (e.g., PIFS) before a triggerframe is received, when SIFS elapsed after the trigger frame isreceived.

Second, the threshold value indicated by the trigger frame correspondsto a decimal value corresponding to bits transmitted via the triggerframe and a unit of the threshold value may correspond to us. Thethreshold value can be configured by a duration based on a value of bitstransmitted by the AP. For example, when the AP transmits ‘0111’, ifduration of a resource (e.g., PPDU or A-MPDU) allocated for the STA orduration of a UL frame (e.g., PPDU or A-MPDU) to be transmitted by theSTA is equal to or less than 7 μs, it may be able to transmit a UL frameirrespective of a CCA value of a specific duration (e.g., PIFS) before atrigger frame is received, when SIFS elapsed after the trigger frame isreceived.

Third, it may define a mapping table between a specific threshold valueand bits transmitted by the AP and transmit a value of the mapping tableindicating the specific threshold value. For example, when a mappingrelation between ‘0111’ and 2000 bytes is defined, the AP transmits‘0111’ and STAs receives the ‘0111’. Among the STAs, if a size/length ofa resource (e.g., PPDU or A-MPDU) allocated for an STA including an idleNAV or a size/length of a UL frame (e.g., PPDU or A-MPDU) to betransmitted by the STA is equal to or less than 2000 bytes, the STAincluding idle NAV may be able to transmit a UL frame irrespective of aCCA value of a specific duration (e.g., PIFS) before a trigger frame isreceived, when SIFS elapsed after the trigger frame is received. As adifferent example, when ‘0111’ is defined by 3 (e.g., μs unit) inadvance and the AP transmits ‘0111’, if duration of a resource (e.g.,PPDU or A-MPDU) allocated for an STA including an idle NAV or durationof a UL frame (e.g., PPDU or A-MPDU) to be transmitted by the STA isequal to or less than 3 μs, the STA including idle NAV may be able totransmit a UL frame irrespective of a CCA value of a specific duration(e.g., PIFS) before a trigger frame is received, when SIFS elapsed afterthe trigger frame is received.

As a different embodiment, when an STA receives a trigger frame, the STAcan always transmit a UL frame when SIFS elapsed immediately after thetrigger frame is received.

As a further different embodiment, among STAs which have received atrigger frame, if NAV of an STA is idle and ACK policy of a UL framecorresponds to No ACK or delayed BA, the STA may be able to transmit aUL frame irrespective of a CCA value of a specific duration (e.g., PIFS)before a trigger frame is received, when SIFS elapsed immediately afterthe trigger frame is received.

As a further different embodiment, when a trigger frame is transmitted,information on whether to perform CCA or whether to reflect CCA can beincluded in the trigger frame. For example, when the AP transmits atrigger frame, it may be able to indicate whether or not a receiving STAperforms CCA (whether to discard CCA result, whether to reflect CCAresult) using z bits (in this case, z is a random positive number) inHE-SIG1, HE SIG2, and MAC header. When the AP indicate STAs not toperform CCA using a trigger frame, an STA including idle NAV may be ableto transmit a UL frame irrespective of a CCA value of a specificduration (e.g., PIFS) before the trigger frame is received, when SIFSelapsed immediately after the trigger frame is received. Or, when the APtransmits a trigger frame, if 1 bit for indicating whether to performCCA in HE-SIG1, HE SIG2, and MAC header is set to ‘1’, an STA includingidle NAV may be able to transmit a UL frame or a shortmanagement/control frame irrespective of a CCA value of a specificduration (e.g., PIFS) before the trigger frame is received, when SIFSelapsed after the trigger frame is received. Or, if NAV is idle and aCCA result is idle, it may be able to perform uplink transmission. Forexample, when the AP transmits a trigger frame, if 1 bit for indicatingwhether to perform CCA in HE-SIG1, HE SIG2, and MAC header is set to‘0’, an STA including idle NAV may be able to transmit a UL frame whensecondary channels are idle during specific duration (e.g., SIFS) beforethe trigger frame is received (when a specific channel is idle in caseof dynamic allocation, when all channels are idle in case of staticallocation).

Recovery Method when Uplink Transmission Fails

In the following, a recovery method or a retransmission method isexplained when transmission/reception of a UL frame triggered by atrigger frame is failed. Following descriptions can be used togetherwith the aforementioned uplink transmission method which is performedafter a trigger frame is received or can be independently used.

(i) If a channel is idle, ii) if a frame is received from a differentSTA belonging to the same BSS, or iii) if a frame is received from anSTA belonging to a different BSS, within specific duration (e.g., PIFS)after a trigger frame is transmitted among UL frames triggered by an APvia a trigger frame, as shown in FIG. 23, the AP can truncate TXOP,which is configured by transmitting a CF-End frame within TXOP limitwhen specific time (e.g., PIFS) elapsed after the trigger frame istransmitted. When STAs receive the CF-End frame from the AP, the STAsreset NAV. If the STAs have data to be transmitted, the STAs may startcontending. i) If a frame is received from a different STA belonging tothe same BSS or ii) if a frame is received from an STA belonging to adifferent BSS, within specific duration (e.g., PIFS) after a triggerframe is transmitted among UL frames triggered by an AP via a triggerframe, it may be able to configure the AP not to perform CF-Endtransmission.

(i) If a channel is idle, ii) if a frame is received from a differentSTA belonging to the same BSS, or iii) if a frame is received from anSTA belonging to a different BSS, within specific duration (e.g., PIFS)after a trigger frame is transmitted among UL frames triggered by an APvia a trigger frame, as shown in FIG. 24, the AP can newly configureTXOP by transmitting a trigger frame when specific time (e.g., PIFS)elapsed after the trigger frame is transmitted.

Among STAs which have received a trigger frame from the AP, an STAincluding idle NAV can transmit a PS-poll, Ack/Block Ack, resourcerequest/buffer status report, CTS, NDP frame, and the like, which arecapable of being considered as a UL frame, according to information ofthe trigger frame, when secondary channels are idle during specificduration (e.g., SIFS) before the trigger frame is received (when aspecific channel is idle in case of dynamic allocation, when allchannels are idle in case of static allocation). Or, the STA includingidle NAV can transmit a PS-poll, Ack/Block Ack, resource request/bufferstatus report, CTS, NDP frame, and the like, which are capable of beingconsidered as a UL frame, irrespective of a CCA value of specificduration (e.g., PIFS) before the trigger frame is received when SIFSelapsed after the trigger frame is received. As a method of indicatingan STA to perform CCA, it may be able to apply the aforementionedproposed method.

(i) If a frame is received from a different STA belonging to the sameBSS or ii) if a frame is received from an STA belonging to a differentBSS, within specific duration (e.g., PIFS) after a trigger frame istransmitted among UL frames triggered by an AP via the trigger frame, itmay be able to configure the AP not to transmit a trigger frame.

FIG. 25 is a block diagram for a configuration of a wireless deviceaccording to one embodiment of the present invention.

An AP 10 can include a processor 11, a memory 12 and a transceiver 13.An STA 20 can include a processor 21, a memory 22 and a transceiver 23.The transceiver 13/23 can transmit/receive a radio signal. For example,the transceiver can implement a physical layer according to IEEE 802system. The processor 11/21 is connected with the transceiver 13/23 andcan implement a physical layer and/or an MAC layer according to IEEE 802system. The processor 11/21 can be configured to perform an operationaccording to various embodiments of the present invention. And, a moduleconfigured to implement operations of the AP and the STA according tothe various embodiments of the present invention is stored in the memory12/22 and the module can be executed by the processor 11/21. The memory12/22 is included in the inside of the processor 11/21 or is installedin the outside of the processor 11/21 and can be connected with theprocessor 11/21 by a well-known means.

For a detailed configuration of the AP and the STA, the items mentionedearlier in various embodiments of the present invention can beindependently applied or two or more embodiments can be applied at thesame time. For clarity, explanation on overlapped contents is omitted atthis time.

The embodiments of the present invention may be implemented throughvarious means. For example, the embodiments can be implemented byhardware, firmware, software, or a combination thereof.

When implemented as hardware, a method according to embodiments of thepresent invention may be embodied as one or more application specificintegrated circuits (ASICs), one or more digital signal processors(DSPs), one or more digital signal processing devices (DSPDs), one ormore programmable logic devices (PLDs), one or more field programmablegate arrays (FPGAs), a processor, a controller, a microcontroller, amicroprocessor, etc.

When implemented as firmware or software, a method according toembodiments of the present invention may be embodied as a module, aprocedure, or a function that performs the functions or operationsdescribed above. Software code may be stored in a memory unit andexecuted by a processor. The memory unit is located at the interior orexterior of the processor and may transmit and receive data to and fromthe processor via various known means.

Preferred embodiments of the present invention have been described indetail above to allow those skilled in the art to implement and practicethe present invention. Although the preferred embodiments of the presentinvention have been described above, those skilled in the art willappreciate that various modifications and variations can be made in thepresent invention without departing from the spirit or scope of theinvention. For example, those skilled in the art may use a combinationof elements set forth in the above-described embodiments. Thus, thepresent invention is not intended to be limited to the embodimentsdescribed herein, but is intended to accord with the widest scopecorresponding to the principles and novel features disclosed herein.

INDUSTRIAL APPLICABILITY

Although various embodiments of the present invention are explainedcentering on IEEE 802.11 system, the embodiments can also be applied tovarious mobile communication systems with an identical scheme.

What is claimed is:
 1. A method of performing uplink transmission, by an STA (station), in a wireless communication system, the method comprising: receiving, by the STA, a trigger frame triggering transmission of the response frame from an AP (Access Point); and transmitting, by the STA, the response frame to the AP, wherein whether to perform a CCA (clear channel assessment) for transmission of the response frame is determined based on a length of the response frame.
 2. The method of claim 1, wherein when the length of the response frame is less than or equal to a threshold, the STA transmits the response frame when a SIFS (short inter-frame space) has elapsed after the trigger frame is received.
 3. The method of claim 1, wherein when the length of the response frame is less than or equal to a threshold, the STA transmits the response frame without performing the CCA.
 4. The method of claim 1, wherein when the length of the response frame is larger than a threshold, the STA transmits the response frame considering a result of the CCA.
 5. The method of claim 1, wherein the response frame is transmitted only if a medium is idle considering both the result of the CCA and a NAV (Network Allocation Vector) count when the length of the response frame is greater than a threshold.
 6. The method of claim 1, wherein the length of the response frame is determined based on a length of a PPDU allocated from the AP for transmission of the response frame.
 7. The method of claim 2, wherein the threshold is determined based on reserved bits of a HE-SIG of the trigger frame,
 8. The method of claim 1, wherein the response frame is a frame related to a block ACK (Acknowledgement) frame.
 9. The method of claim 1, wherein the response frame is an uplink multi-user Block ACK (Acknowledgement) frame.
 10. An STA (station) performing uplink transmission in a wireless communication system, the STA comprising: a transceiver; and a processor, the processor configured to receive a trigger frame triggering transmission of the response frame from an AP (Access point), wherein the processor is further configured to transmit the response frame to the AP, wherein whether to perform a CCA (clear channel assessment) for transmission of the response frame is determined based on a length of the response frame. wherein when the length of the response frame is less than or equal to a threshold, the STA transmits the response frame when a SIFS (short inter-frame space) has elapsed after the trigger frame is received.
 11. The STA of claim 10, wherein when the length of the response frame is less than or equal to the threshold, the processor transmits the response frame without performing the CCA.
 12. The STA of claim 10, wherein when the length of the response frame is larger than the threshold, the processor transmits the response frame considering a result of the CCA.
 13. The STA of claim 10, wherein the response frame is transmitted only if a medium is idle considering both the result of the CCA and a NAV (Network Allocation Vector) count when the length of the response frame is greater than a threshold.
 14. The STA of claim 10, wherein the length of the response frame is determined based on the length of the PPDU allocated from the AP for transmission of the response frame.
 15. The STA of claim 11, wherein the threshold is determined based on reserved bits of a HE-SIG of the trigger frame,
 16. The STA of claim 10, wherein the response frame is a frame related to a block ACK (Acknowledgement) frame.
 17. The STA of claim 10, wherein the response frame is an uplink multi-user Block ACK (Acknowledgement) frame. 